This invention relates to the adhesive joining of optical fibers, and especially to the adhesive joining of optical fibers which have been optically aligned.
In commonly assigned U.S. patent application No. 258,079, filed Apr. 27, 1981, now abandoned, continuation-in-part 370,321 now abandoned filed Apr. 21, 1982, and continuation-in-part 437,053 filed Oct. 27, 1982, the disclosures of which are incorporated herein by reference, there is disclosed a technique for the adhesive joining of optical fibers. The fibers are butted end-to-end within a tube containing index-matching adhesive, optically aligned by measuring the light intensity passing from one fiber to the other and maximizing that intensity, and the adhesive is then caused to harden. A preferred adhesive is a photopolymerizable adhesive, i.e. one that will harden on exposure to ultraviolet light.
It has been discovered, however, that shrinkage and flow of the adhesive as it sets tend to cause a loss of orientation of the fiber ends with respect to each other, which may result in an unacceptably high loss at the joint.
This adverse effect is a consequence of the adhesive being in a tube: polymerization of a drop of adhesive between two fiber ends but otherwise unconstrained does not cause a problem. However, in a tube, the surface interface between the adhesive and the tube inner wall restricts the freedom of the adhesive to shrink without coming under stress. Eventually the stress is relieved by cracking or void formation in the adhesive, flow of the adhesive, or separation at an adhesive surface.
Because the fibers are flexible, it is not believed possible to rely on simply holding the fibers to prevent their movement: although the parts which are held remain still, the forces exerted on the ends by the shrinking adhesive are quite sufficient to cause relative movement.
It has been proposed in British Published Patent Application No. 2,039,379A, the disclosure of which is incorporated herein by reference, to cure a photopolymerizable adhesive about an optical fiber which is to be aligned with a lens. This curing comprises first curing the adhesive adjacent the lens and the end of the fiber, and then curing the remaining adhesive by gradually exposing it to light. The technique proposed in that application relies on the ability to symmetrically cure the adhesive about the fiber end to avoid loss of alignment.
U.S. Pat. No. 4,201,618 (Lewis) disclosures an apparatus designed to achieve the symmetric curing of a photopolymerizable adhesive used to join the ends of optical fibers. The adhesive is contemplated as being unconstrained (not in a tube), and a uniformly stressed bonded connection is said to be formed.